There is a fairly long history behind this.
The distinction between capital and lower case letters came quite late even in the history of alphabetic writing (and alphabetic writing was invented some 2200 years after the Mesopotamian invention of writing itself). Ancient Greek was originally written in scripta continua (all capitals, no spaces between words); e.g. the opening of Homer’s Iliad would have been written “SINGOGODDESSTHEANGEROFACHILLES”. Such innovations as word divisions and accent marks were developed by Hellenistic scholars, making poems easier to read for young children and non-native speakers of Greek, but gradually becoming standard. Miniscule, a type of writing similar to modern cursive (with upper and lower case and frequent ligatures) was an early medieval development. Standards of capitalization, with the first lines of poems being capitalized, and the use of “title case” for poetic titles was a comparatively late feature, developed gradually in the age of print.
Many modern poets, most notably e. e. cummings, considered capitalization one of the standards against which they were rebelling and began to revert to the sort of rhetorical rather than conventional capitalization more common in the 18th century. Although in some ways this movement is described as a fascination with typography and the poem as a visual object, it can, especially in the case of poets like Bukowski, be seen as a rejection of purely visual conventions in poetics and a return to poetry as an oral form.